Photocatalytic mineralization of benzene over gold containing titania nanotubes: Role of adsorbed water and nanosize gold crystallites

Well aligned titania nanotubes, with surface area of ∼150 m2 g−1 and average pore diameter of ∼5 nm, have been synthesized by subjecting a mesoporous TiO2 sample to alkaline treatment followed by calcination at different temperatures. Composite catalysts with gold particles dispersed in these nanotubes were also synthesized and their structural, morphological, optical and photocatalytic properties were examined. The catalytic activity of TiO2 for UV-mediated photo-oxidation of benzene was found to be affected by several factors, such as the sample texture, presence of adsorbed water and gold crystallites. The overall conversion of benzene to form CO2 followed a trend: Au/nanotube > TiO2 nanotubes > mesoporous TiO2. In situ IR spectroscopy revealed that the adsorption and reaction of benzene molecules gave rise to formation of certain phenolic species over TiO2, while the temperature-programmed desorption (TPD) study showed that the Au/TiO2 interfaces serve as distinct sites for the adsorption and activation of oxygen molecules. It is suggested that certain hydroxyl and oxygen ion radicals produced under UV-irradiation may promote the deep oxidation of surface phenolic species and phenoxyl (ArOradical dot) type transient radicals.